A nacelle generally has a tubular structure comprising an air intake upstream of the turbojet, a middle section intended to surround the turbojet case, a downstream section housing thrust reverser means and intended to surround the gas generator, as well as, most often, a jet nozzle whereof the outlet is situated downstream of the turbojet engine.
Nacelles are intended to house a single- or dual-flow turbojet engine.
In general, part of the noise caused by an aircraft turbojet is generated by the ejection speed of the burnt gases, coming from the jet nozzle of the nacelle, and more specifically by the meeting between the burnt gases thus expelled by the jet nozzle and the surrounding outside air.
Reducing the noise, in particular during takeoff phases of the aircraft, constitutes one of the high priority objectives in the design of turbojet engines and the associated nacelles.
To that end, it is known to make part of the inner wall of the intake structure of the nacelle in the form of a cellular structure in which the power of the sound waves is damped.
To that end, also known are nacelles whereof the downstream section has cutouts in the form of chevrons to better mix the flow of burnt gases from the turbojet with the surrounding outside air, and to thereby reduce the noise annoyances generated by the turbojet engine.
However, these chevrons are generally fixed, which has an impact on the aircraft's performance.
For example, the amount of fuel consumed relative to a smooth jet nozzle increases at a comparable thrust.
For that reason, devices have been proposed making it possible both to reduce the aeroacoustic noise from the turbojet engine, in particular in the takeoff phase, substantially without damaging the performance of the aircraft during the other flight phases of the aircraft.
This is for example the case of the device presented in document FR 2 868 131, which proposes a nacelle jet nozzle with variable geometry adapted for a dual-flow turbojet engine. “Dual-flow turbojet engine” refers to a turbojet capable of generating, via the rotating fan blades, a hot air flow (or primary flow) coming from the combustion chamber of the turbojet engine, and a cold air flow (or secondary flow) that circulates outside the turbojet through a jet formed between a fairing of the turbojet and an inner wall of the nacelle, the two air streams ejected from the turbojet through the rear of the nacelle being made to mix with the surrounding outside air flow.